The present invention relates to mandrels for producing connecting sockets at the end of thermoplastic pipes, particularly a mandrel with retractable segments permitting the shaping of an annular groove within a cylindrical pipe.
It is known for the connection of two ends of thermoplastic pipes to plastically shape one end of one pipe, so that the internal diameter is widened to such an extent that it corresponds to or is slightly larger than the external diameter of the other pipe. This method can advantageously be supplemented by an annular groove within the first widened end, which is provided for receiving a gasket.
To this end mandrels with a shaping surface have been constructed whose outer contours correspond to the inner surface of the pipe end to be widened and over which is firstly forcefitted a heat-softened end of a thermoplastic pipe. After cooling and hardening the thermoplastic material it is indispensible to ensure the retraction of the mandrel surfaces which have permitted the construction of the annular groove in order to permit the removal of the thus shaped pipe.
In general the known mandrels with retractable segments can be subdivided into two different categories having two rows of retractable segments which can assume an extended position in which the radially outer surface of the segments of both rows, arranged in alternating manner, form a continuous annular flange which projects radially from the shaping surfaces of the mandrel, and a retracted position in which the distance from the radially outermost surface to the mandrel axis is smaller than the radius of the shaping surfaces, the segments of both rows being axially displaceable, depending on the means used for actuating said first and second retractable segment rows.
There are firstly mandrels in which the retractable segments are pivotally mounted on a shaft which is perpendicular to the mandrel axle. In this case it is simple to move the retractable segments, leading to a simplification of the actuation means and ensuring reliable operation.
However, every point on the radially outer surface of the retractable segments describes a circular arc during the extension and retraction, which makes it impossible to construct annular grooves with a rectangular or some other complicated shape.
In order to obviate the above deficiencies mandrels have been constructed in which the retractable segments are successfully radially and axially moved. To retract the segments the segments of one row are radially retracted and then axially displaced. This displacement then permits the retraction of the second row of segments. In this way it is possible to produce annular grooves with different shapes.
However, in the case of mandrels with retractable segments of this type, the actuating means are very complicated and comprise dovetail guides and cones. This often leads to faulty operation, particularly due to the thermal stressing to which the mandrel is exposed.